Whenever a spy's cover is revealed, a chain of setbacks ensues. Foreign intelligence services then review everything they know about the undercover officer who was operating in their country. Such a review can lead not only to the discovery of informants who may have been recruited by the outed CIA officer but also to an understanding of the practices and techniques used by an undercover figure such as Plame, who posed as a businesswoman abroad. After one undercover CIA officer is exposed, others inevitably have a harder time persuading potential sources to pass secrets about their government's -- or their terrorist network's -- plans and capabilities.
Once before, Plame was caught up in a case illustrating how costly it can be for a CIA officer to be in danger of having her cover exposed. The agency called Plame home in 1997 in fear that Aldrich Ames, the notorious Soviet mole inside the CIA, had revealed her true identity to his KGB handlers. At least 10 people were executed by the Soviets as a consequence of Ames's disclosure of CIA identities.
I just can't say it strongly enough. Disclosure of Plame's identity was so treasonous I don't know why we're not in the streets with torches and pitchforks.